Monthly Archives: September 2016

We all know that search for errors in geometries can be quite a journey. One of the errors we need to fix is the presence of spikes in our geometries. One way to determine the location of those spikes is to determine the angles and check if them are smaller than a predefined threshold.

Ok them, one way to tackle this problem is to load your data into a PostGIS layer and use the available ST functions.

In this post I’ll show you guys a SQL query to solve this.

Let’s suppose we have geometries with problems like these here:

To solve this problems we will use the following ST PostGIS functions:

Someone has given you a logical model with over 500 tables to implement it in PostgreSQL, where to begin? Should I cry first? Should I just start typing create table statements like there is no tomorrow? No! I present you pgModeler!

pgModeler is a modelling open software developed by Raphael Araújo e Silva and it allows you to build a database just using a nice graphical interface and after you finish your design, you can just deploy it and there you go! Your database is ready!

For instance, suppose you have a table called road with an text attribute “name”, geometry column “geom” with type MultiLinestring, with epsg 4326. To implement this table you just have to click around and there you go, you have the following table:

To deploy this, just go to export and choose between .png, sql or direct deploy into postgres. The sql for the table above is:

Have you ever needed to explode multi geometry layer into a single geometry layer, using in each new geometry the attributes of the original multi one? If you were working with FME, for instance, you basically would just use the transformer Deaggregator. Let’s learn how to solve this problem with python and QGIS!

The following code snippet teaches you how to work with QgsVectorLayers, it’s attributes and how to manipulate geometries.

from qgis.core import QgsVectorLayer, QgsFeature, QgsMapLayerRegistry
#fill in your input layer name. In this example, our inputLyrName is input_layer
inputLyrName = 'input_layer'
inputLyr = QgsMapLayerRegistry.instance().mapLayersByName(inputLyrName)[0]
#fill in your output layer name. In this example, our outputLyrName is output_layer
outputName = 'output_layer'
outputLyr = QgsMapLayerRegistry.instance().mapLayersByName(outputLyrName)[0]
#tests type of output: if it is a multi parted geometry or
#a single parted geometry
if outputLyr.wkbType() in [QGis.WKBPoint, QGis.WKBLineString, QGis.WKBPolygon]:
isMulti = False
else:
isMulti = True
outputLyr.startEditing()
addList = []
for feat in inputLyr.getFeatures():
#gets all parts of geometry as an individual single geometry
parts = feat.geometry().asGeometryCollection()
#checks if it isMulti, if it is, convert each
#part in geometryCollection to multi
if isMulti:
for part in parts:
part.convertToMultiType()
#for each part, get original set of attribute and create a new feat
#with this set
for i in range(0,len(parts)):
#new feature constructor. newFeat has all atributes of feat
newFeat = QgsFeature(feat)
#set geometry with part
newFeat.setGeometry(parts[i])
#get field id and get defaultValue from provider
idx = newFeat.fieldNameIndex('id')
newFeat.setAttribute(idx,provider.defaultValue(idx))
addList.append(newFeat)
outputLyr.addFeatures(addList,True)
outputLyr.commitChanges()

We all know that GRASS is a great GIS software. Combined with QGIS it is even more great!

Using GRASS from within QGIS is very useful to deal with daily GIS problems. Everyone that works with geospatial data knows how annoying is to clean up geometries full of errors. The manual process demands lots of time and we can always forget something in the end. Do this kind of job automatically is faster and safer.

Let’s se how to do this using pyqgis. Imagine that we have a database layer like this:

A good way to clean problems like those shown above and at the same time solve snapping problems is to use the following tools in v.clean.advanced provided by GRASS: